Injection molding (IM) is an approach for fabricating articles from powders of the constituents. In injection molding, powder particles are mixed with a binder and usually other constituents, such as a lubricant, so as to make a flowable feedstock mixture. The feedstock mixture is injected into a mold under pressure using an injection-molding machine similar to those used to injection mold plastics. The injected mass, termed a “green” article, is removed from the mold. Most of the constituents of the green article other than the particles, and specifically the binder and lubricant, are largely removed from the green article by a suitable process such as heating to vaporize the ingredients or solvent extraction, leaving a relatively fragile body termed a “brown” article that has the molded shape but little mechanical strength. This terminology of “green” article and “brown” article is widely used in the industry and is also utilized herein. The brown article is then consolidated, typically by sintering, to produce the final article.
In one variety of injection molding, the particles are of a metallic alloy, and the process is termed metal injection molding (MIM). In MIM, gas-atomized or water-atomized metal alloy powder is mixed with the binder and other constituents to make the feedstock. The resulting article is formed of the metallic alloy.
Articles may be produced by MIM to precise dimensional tolerances. The green article is oversize, and then shrinks during the subsequent process steps to the required dimensions of the final article. The shrinkage is predictable, so that MIM may be used to make complex metal alloy articles to precise dimensional requirements. Articles produced by MIM are typically not used for demanding applications requiring high mechanical properties, because there is typically some porosity left in the article after sintering.
Injection molding generally and MIM specifically are widely used, but there is a need to modify its current approach to improve the properties of the final article and reduce the cost of the final article. The present invention fulfills this need, and further provides related advantages.